Solution processed quantum dot (QD)/nanocrystal (NC) solar cells have emerged in the last decade as a very promising technology for third-generation thin film photovoltaics due to their low cost and high energy harnessing potential. A variety of quantum dot solar cell architectures have been developed, however, all reports so far have relied on the employment of bulk-like thin films of colloidal quantum dots. This has limited the material availability to nanomaterials that exhibit the favorable property of long carrier lifetime evidenced in Pb-based and Cd-based quantum dots.
Solution processed inorganic solar cells based on colloidal quantum dots and nanocrystals have attracted tremendous interest in view of their potential for panchromatic solar harnessing from the visible to the infrared and their low fabrication cost associated with solution processing (Luther, J. M. et al. Schottky solar cells based on colloidal nanocrystal films. Nano Lett. 8, 3488-3492 (2008); Ma, W. et al. Photovoltaic performance of ultrasmall PbSe quantum dots. Acs Nano 5, 8140-8147 (2011); Pattantyus-Abraham, A. G. et al. Depleted-heterojunction colloidal quantum dot solar cells. Acs Nano 4, 3374-3380 (2010); Brown, P. R. et al. Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer. Nano Lett. 11, 2955-2961, (2011)). Solution processed inorganic thin-film solar cells consist of either nanocrystalline bulk semiconductors based on CdTe (Jasieniak, J., MacDonald, B. I., Watkins, S. E. & Mulvaney, P. Solution-processed sintered nanocrystal solar cells via layer-by-layer assembly. Nano Lett. 11, 2856-2864 (2011)), CIGS (Panthani, M. G. et al. Synthesis of CuInS2, CuInSe2, and CuInxGa1-xSe2 (CIGS) nanocrystal “Inks” for printable photovoltaics. J. Am. Chem. Soc. 130, 16770-16777 (2008)) or CZTS (Guo, Q., Hillhouse, H. W. & Agrawal, R. Synthesis of Cu2ZnSnS4 nanocrystal ink and its use for solar cells. J. Am. Chem. Soc. 131, 11672-11673 (2009)) or nanocrystalline quantum confined systems based on Pb(S, Se) Luther, J. M. et al. Schottky solar cells based on colloidal nanocrystal films. Nano Lett. 8, 3488-3492 (2008); Ma, W. et al. Photovoltaic performance of ultrasmall PbSe quantum dots. Acs Nano 5, 8140-8147 (2011); Pattantyus-Abraham, A. G. et al. Depleted-heterojunction colloidal quantum dot solar cells. Acs Nano 4, 3374-3380 (2010); Brown, P. R. et al. Improved current extraction from ZnO/PbS quantum dot heterojunction photovoltaics using a MoO3 interfacial layer. Nano Lett. 11, 2955-2961, (2011); Luther, J. M. et al. Stability assessment on a 3% bilayer PbS/ZnO quantum dot heterojunction solar cell. Adv. Mater. 22, 3704-3707 (2010); Tang, J. et al. Quantum dot photovoltaics in the extreme quantum confinement regime: the surface-chemical origins of exceptional air- and light-stability. Acs Nano 4, 869-878 (2010)) and Cd(Se, S, Te) (Gur, I., Fromer, N. A., Geier, M. L. & Alivisatos, A. P. Air-stable all-inorganic nanocrystal solar cells processed from solution. Science 310, 462-465, (2005)) quantum dots. Quantum dots offer the potential of bandgap tunability for more efficient solar harnessing (Wang, X. et al. Tandem colloidal quantum dot solar cells employing a graded recombination layer. Nature Photon. 5, 480-484 (2011); Choi, J. J. et al. Solution-processed nanocrystal quantum dot tandem solar cells. Adv. Mater. 23, 3144-3148 (2011). However, only a small fraction of available semiconductor materials has been exploited for the development of third-generation thin-film solar cells. An important criterion in the selection of material candidates for solar cells is the carrier lifetime: carrier lifetime should be long enough to allow for efficient diffusion and drift of photogenerated carriers to the metal contacts before they recombine. In the case of solution processed quantum dot devices, Pb(S, Se) quantum dots outperform any other material to date thanks to the carrier lifetime offered from favorably shallow traps that allow for efficient exciton dissociation and recombination quenching (Barkhouse, D. A. R., Pattantyus-Abraham, A. G., Levina, L. & Sargent, E. H. Thiols passivate recombination centers in colloidal quantum dots leading to enhanced photovoltaic device efficiency. Acs Nano 2, 2356-2362 (2008)). This has been exploited in various solar cell structures developed to date that rely on the existence of a bulk-state thin film of colloidal quantum dots where carrier photogeneration and bipolar transport occur in the same medium. This has yielded devices with efficiencies steadily increasing from 2% (Luther, J. M. et al. Schottky solar cells based on colloidal nanocrystal films. Nano Lett. 8, 3488-3492 (2008)) up to 6% (Tang, J. et al. Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Mater. 10, 765-771 (2011)) over the past years as a result of increasing carrier mobility and depletion width followed by complex chemistry surface engineering that yields low doping concentration (Tang, J. et al. Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Mater. 10, 765-771 (2011)). However this elaborate control of carrier doping has thus far been limited mainly in PbS QDs, a well-studied material thanks to its favorably long carrier lifetime.
A bulk heterojunction concept has been employed in polymer-based solar cells as the sole way for exciton dissociation and diffusion to compete with ultra-fast recombination and short exciton diffusion length (Yu, G., Gao, J., Hummelen, J. C., Wudl, F. & Heeger, A. J. Polymer photovoltaic cells-enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 270, 1789-1791, (1995); Oosterhout, S. D. et al. The effect of three-dimensional morphology on the efficiency of hybrid polymer solar cells. Nature Mater. 8, 818-824, (2009)). Efforts to reproduce the benefits of this structure in polymer-nanocrystal hybrid solar cells has not fulfilled the expectations due to the challenges of achieving the appropriate nanomorphology for efficient exciton dissociation and carrier transport in these heterogeneous nanocomposites. It was only recently that polymer-grafted quantum dot hybrid solar cells with controlled nanostructure led to efficiency improvement in hybrid polymer-quantum dot solar cells (Ren, S. et al. Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires. Nano Lett. 11, 3998-4002 (2011)).
The development of an inorganic bulk heterojunction based on PbS QDs and titania meso-structured electrodes has been reported in a depleted heterostructure. However the reported devices relied on the employment of a meso-structured electrode that does not contribute in light absorption (Barkhouse, D. A. R. et al. Depleted bulk heterojunction colloidal quantum dot photovoltaics. Adv. Mater. 23, 3134-3138 (2011)). The scale of the reported heterojunction was on the order of 100 nm resulting in modest increase in power conversion efficiency compared to their bilayer counterparts (Pattantyus-Abraham, A. G. et al. Depleted-heterojunction colloidal quantum dot solar cells. Acs Nano 4, 3374-3380 (2010)).
US-2011/0146766-A1 discloses solar cells comprising a nanocrystal film of a single material such as a quantum dot material, a colloidal nanocrystal material or a combination thereof, arranged between a first and a second electrode.
US-2006/0243959-A1 discloses three-dimensional bicontinuous heterostructures including two interpenetrating, spatially continuous layers including protrusions or peninsulas but no islands. A first of the two layers is made of a first material and the second of the two layers is made of second material. One of the materials includes visible and/or infrared semiconducting quantum dot nanoparticles and one of the materials is a hole conductor and the other an electron conductor.
US-2009/0217973-A1 discloses a photovoltaic device comprising an inorganic photoactive layer disposed between electrode layers and comprising two heterojunctioned populations of nanostructures of different inorganic materials.
The photovoltaic efficiency of the thin-film based solution processed quantum dot (QD)/nanocrystal (NC) solar cells known from prior art is still improvable and, further, the techniques used to prepare such films is rather cumbersome.